Thirty grams is about an ounce...for oatmeal just a little less than a half cupful dry (which is 40 grams.)
Go into you pantry and find something like cookies. The nutrition list will say something like Serving Size 3 cookies and give the weight in grams...throw the three cookies (or whatever is the serving size) on your scale and see if you get the same weight.
edit: AHA, just thought of the easiest way. Weigh some U.S. pennies 1962 through 1982...they are almost EXACTLY 3 grams each.
And the newer ones: 2.5 grams each.
If you're RICH, dimes weigh 2.27 grams
...so 10 of them should weigh either 22 or 23 grams if your scale is a good one.